Evolution
ns, and is responsible for the diversity of life we see today.]] In biology, evolution is change in the genetic material of a population of organisms from one generation to the next. Though the changes produced in any one generation are small, differences accumulate with each generation and can, over time, cause substantial changes in the organisms. This process can culminate in the emergence of new species. Indeed, the similarities between organisms suggest that all known species are descended from a common ancestor (or ancestral gene pool) through this process of gradual divergence. The basis of evolution is the genes that are passed on from generation to generation; these produce an organism's inherited traits. These traits vary within populations, with organisms showing heritable differences (variation) in their traits. Evolution itself is the product of two opposing forces: processes that constantly introduce variation, and processes that make variants become more common or rare. New variation arises in two main ways: either from mutations in genes, or from the transfer of genes between populations and between species. In species that reproduce sexually, new combinations of genes are also produced by genetic recombination, which can increase variation between organisms. Two major mechanisms determine which variants will become more common or rare in a population. The first is natural selection, a process that causes helpful traits (those that increase the chance of survival and reproduction) to become more common in a population and causes harmful traits to become more rare. This occurs because individuals with advantageous traits are more likely to reproduce, meaning that more individuals in the next generation will inherit these traits. Over many generations, adaptations occur through a combination of successive, small, random changes in traits, and natural selection of the variants best-suited for their environment. The second major mechanism driving evolution is genetic drift, an independent process that produces random changes in the frequency of traits in a population. Genetic drift results from the role that chance plays in whether a given trait will be passed on as individuals survive and reproduce. Evolutionary biologists document the fact that evolution occurs, and also develop and test theories that explain its causes. The study of evolutionary biology began in the mid-nineteenth century, when studies of the fossil record and the diversity of living organisms convinced most scientists that species changed over time. However, the mechanism driving these changes remained unclear until the theories of natural selection independently discovered by Charles Darwin and Alfred Wallace. Darwin's landmark work On the Origin of Species of 1859 brought the new theories of evolution by natural selection to a wide audience. . Related earlier ideas were acknowledged in Darwin's work soon led to overwhelming acceptance of evolution among scientists. Joint statement issued by the national science academies of 67 countries, including the United Kingdom's Royal Society from the world's largest general scientific society In the 1930s, Darwinian natural selection was combined with Mendelian inheritance to form the modern evolutionary synthesis, which connected the units of evolution (genes) and the mechanism of evolution (natural selection). This powerful explanatory and predictive theory directs research by constantly raising new questions, and it has become the central organizing principle of modern biology, providing a unifying explanation for the diversity of life on Earth. Evidence for evolution Scientific evidence for evolution comes from many aspects of biology, and includes fossils, homologous structures, and molecular similarities between species' DNA. Fossil record Research in the field of paleontology, the study of fossils, supports the idea that all living organisms are related. Fossils provide evidence that accumulated changes in organisms over long periods of time have led to the diverse forms of life we see today. A fossil itself reveals the organism's structure and the relationships between present and extinct species, allowing paleontologists to construct a family tree for all of the life forms on earth. Modern paleontology began with the work of Georges Cuvier (1769–1832). Cuvier noted that, in sedimentary rock, each layer contained a specific group of fossils. The deeper layers, which he proposed to be older, contained simpler life forms. He noted that many forms of life from the past are no longer present today. One of Cuvier’s successful contributions to the understanding of the fossil record was establishing extinction as a fact. In an attempt to explain extinction, Cuvier proposed the idea of “revolutions” or catastrophism in which he speculated that geological catastrophes had occurred throughout the earth’s history, wiping out large numbers of species. Cuvier's theory of revolutions was later replaced by uniformitarian theories, notably those of James Hutton and Charles Lyell who proposed that the earth’s geological changes were gradual and consistent. However, current evidence in the fossil record supports the concept of mass extinctions. As a result, the general idea of catastrophism has re-emerged as a valid hypothesis for at least some of the rapid changes in life forms that appear in the fossil records. A very large number of fossils have now been discovered and identified. These fossils serve as a chronological record of evolution. The fossil record provides examples of transitional species that demonstrate ancestral links between past and present life forms. One such transitional fossil is Archaeopteryx, an ancient organism that had the distinct characteristics of a reptile (such as a long, bony tail and conical teeth) yet also had characteristics of birds (such as feathers and a wishbone). The implication from such a find is that modern reptiles and birds arose from a common ancestor. References External links ;General information * [http://www.newscientist.com/topic/evolution Everything you wanted to know about evolution by New Scientist] * Howstuffworks.com — How Evolution Works * National Academies Evolution Resources * Synthetic Theory Of Evolution: An Introduction to Modern Evolutionary Concepts and Theories * Understanding Evolution from University of California, Berkeley ;History of evolutionary thought * The Complete Work of Charles Darwin Online * Understanding Evolution: History, Theory, Evidence, and Implications ;On-line lectures *What Genomes Can Tell Us About the Past - lecture by Sydney Brenner *The Origin of Vertebrates - lecture by Marc Kirschner Category:Evolution *